1. Technical Field
The present invention relates to a pressure sensor, especially to a preloaded pressure sensor module (PPSM) and its applications.
2. Description of Related Art
FIG. 1 shows a prior art flat piezoresistor device
FIG. 1 shows a flat piezoresistor device 10 which has a top substrate 111, a top electrode layer 112 configured under the top substrate 111, a top piezoresistive layer 113 configured under the top electrode layer 112; and a bottom substrate 121, a bottom electrode layer 122 configured on the bottom substrate 121, a bottom piezoresistive layer 123 configured on the bottom electrode layer 122. Further, a pair of spacers 114 is configured in between the top electrode layer 112 and the bottom electrode layer 122 such that a space 13 is formed in between the top piezoresistive layer 113 and the bottom piezoresistive layer 123. The top electrode layer 112 electrically couples to a first electrode, for example, positive (+) end of a control unit 15; and the bottom electrode layer 122 electrically couples to a second electrode, for example, negative (−) end of the control circuit 15.
FIG. 2 shows the flat piezoresistor device of FIG. 1 being pressed.
FIG. 2 shows that the center of the flat piezoresistor of FIG. 1 is pressed by a user. The circuit passes through the top electrode layer 112 and the bottom electrode layer 122 is closed when the top piezoresistive layer 113 touches the bottom piezoresistive layer 123. The signal is, however, unstable in the initial stage when the two piezoresistive layers 113, 123 touch each other due to initial unstable surface contact as illustrated in FIG. 3.
FIG. 3 shows the electrical characteristic for the prior art of FIG. 2
Y-axis shows output conductivity, X-axis shows the pressure force exerted against the flat piezoresistor device of FIG. 1. Line L1 shows the electrical characteristics for Output Conductivity versus Pressure. The output conductivity is zero during the pressure ranges between P00˜P10, where the two piezoresistive layers 113, 123 are not in contact.
The output conductivity is unstable when the pressure force at a point ranges between P10 and P11. This is because of the unstable initial contact of the two piezoresistive layers 113 and 123. Line L1 shows linearity for pressure measurement at the points after P11 as the characteristic becomes adequate for application of pressure measurement. For a real case, a detection threshold DT is usually set on a point after P11, for example at P12 to assure measurement quality of the sensor. The pressure force at P12 is the minimum detectable limit for the flat pressure sensor 10. The pressure force at P12 is also known as an activation force for the sensor 10.
The disadvantage for the prior art flat pressure sensor of FIG. 1 is that the activation force can vary among the sensors in mass production. To produce the prior art sensor illustrated in FIG. 1, the manufacturer faces a tradeoff between detection limit and production yield, as aggressively setting a relatively lower detection limit responsive to a relatively lighter applied pressure may reduce production yield.